To build Openbox from git you need:

A C Compiler (GNU GCC 3.2+ suggested)
GNU Gettext 0.14.4
GNU Autoconf 2.50+
GNU Automake 1.9 (no more, no less)
GNU Libtool
Xlib library/headers (devel package)
Pkg-Config
Glib 2.0+ library/headers (devel package) (http://www.gtk.org)
libxml2 2.0+ library/headers (devel package)
Pango 1.10+ library/headers (devel package)
cvs

Optionally you may want:
X Cursor library/headers (devel package)
Startup Notification library/headers 0.8+ (devel package)

We recommend the latest versions of all these packages.

Do the following to build and install Openbox in git:

% ./bootstrap
% ./configure
% make
su to root and
% make install
or
% sudo make install

Don't try running it from the openbox/ directory without installing, it won't work.
It needs to be installed before it is run.

The following commands will be available: openbox-session,
  openbox-gnome-session, openbox-kde-session, and openbox.

See the man pages for details about them.  If you want to run Openbox on its
  own, you probably want to run "openbox-session".



----
In debian unstable, you want these packages:
gcc
gettext
automake1.9
autoconf
libtool
libpango1.0-dev
pkg-config
libglib2.0-dev
libxml2-dev
libxcursor-dev
libstartup-notification0-dev
xlibs-dev

tip: run "update-alternatives --config automake" and select automake1.9 before
  running ./configure

----